Continuity mistake: When Mongo first comes into town and the man on the horse says "You can't park that animal here," Mongo punches the horse and its rider, and the horse falls. Before the cut to the close up of the man (obviously the stunt double), his head is facing toward the horse's head. After the cut to the close up of the man, his body is facing the opposite direction.
Continuity mistake: The scene where the gang stampedes through Rockridge, a thug on horseback lassos a man who is dragged down with the rope slipping down around his feet. As he's dragged foot first through the mud, he says "Well, that's the end of this suit" only now, the rope is up around his chest and under his arms and he's being pulled in a head first direction.
Continuity mistake: When the Sheriff comes into Lili's dressing room, she has her left hand resting on the desk.When she takes the rose you can tell her left arm is down to her side now. When she tells the sheriff to have a seat, she only raises her right arm but in the next shot her left arm is raised with her hand in the desk again.